The present invention relates to an endoprosthesis, in particular a hip joint prosthesis, provided with a hollow shaft being implantable in a hollow space of a bone, being of a tubular and curved shape and diminishing with respect to its diameter and material cross-section.
Prosthesis shafts are fixed into the pre-rasped hollow space of a bone during implantation by cementing or, for cement-free implantation, they are conical or wedge-shaped in the direction away from the joint, and are driven into the hollow space so that they are fixed to the bone by pressural friction.
Such a prosthesis is described in DE - A 29 33 237. The local elasticity and rigidity of the prosthesis has been adapted to conform with that of the surrounding bone. The adaptation only occurs in the form of the material cross-sectional widths and their relation to the forces acting in the longitudinal axis of the prosthesis. The cross-section and the wall thickness diminish continuously from the top to the bottom of the shaft. A drawback of this arrangement is that the deformations which occur due to forces not acting in the longitudinal axis do not react in the same way as the deformations in the surrounding bone area. Micromovements and a possible loosening of the prosthesis are the results.
Another prosthesis shaft with an essentially rectangular cross-section is described in EP-A-0 243 585. Openings are provided in the two areas on the longer sides of the cross-section (dorsal and ventral) to reduce the shear forces which occur in the contact area between the prosthesis and the bone when they are strained and deflect. A weighty drawback of this arrangement is that there is a relatively high areal pressure between the shaft and the bone on the two shorter sides of the cross-section (medial and lateral). Additionally, as there is no physiological adaptation of the rectangular cross-section to that of the natural marrow zone, there are stress peaks in the region of the Adams bend. The differing right- and left-sided geometries of the femural marrow zone have also not been taken into consideration. Although the flex bility of the prosthesis has been increased due to the openings provided in a plane parallel to the main bending plane--as already practiced in the previously discussed prior art DE-A- 29 33 237--this still is not comparable with that of the surrounding bone area.